1. Field of Invention
The present invention relates to an archery bow, and more particularly to a compound bow with a mechanical mechanism that is drawn to a full drawn position a plurality of times with a greater storage of energy and with a lower draw force than a standard compound bow that requires only a single draw.
2. Description of Prior Art
Compound bows are often used in archery due to their favorable force-draw curve for draw force versus draw length. They allow the archer to draw the draw string back with an increasing draw weight until a peak weight is attained and thereafter the draw weight drops to a minimum draw weight at full draw. This drop off in draw force is referred to as let-off and allows the archer to hold the draw string at full draw for an extended period until the appropriate time to launch the arrow. The mean draw weight applied by the archer lies between the peak draw weight and the minimum draw weight. To obtain the favorable force-draw curve and let-off found in compound bows typically a cam-shaped pulley is mounted at the end of each bow limb to provide the draw string with a lever arm that is greatest in the full drawn position. The cam-shaped pulleys also require syncronization with each other such that the lever arm that they offer to the draw string is the same throughout the launch period for each of the bow limb mounted cam-shaped pulleys.
As the archer applies a draw force to the draw string for a draw length, he imparts an amount of energy equal to the multiplication product of the force and draw length. This energy is stored as potential energy of deformation in the flexible bow limbs as they are deformed from a static state to a dynamic or final state of deformation. When the draw string is released, a return launch force is applied by the draw string onto the arrow causing it to accelerate reaching a final launch velocity such that a greater mean draw force will result in a greater mean return launch force being applied to the arrow during launch and a greater launch velocity. The deformation energy stored in the flexible limbs in bringing the draw string back to a full draw is thereby converted into kinetic energy of the launched arrow generating this arrow velocity. The greater the amount of mean draw force and hence stored deformation energy in the flexible bow limbs, the greater will be the velocity of a particular arrow that is launched. A higher arrow velocity is advantageous since it results in a lower trajectory for the arrow during flight and hence allows the archer to gain accuracy at a larger distance.
A standard compound bow that is 100% efficient would therefore impart a kinetic energy to a launched arrow that is equivalent to the energy imparted by the archer during a single full draw stroke. A compound bow, however, typically contains dynamic viscosity energy losses associated with the movement of the limbs, cams, pulleys, and other moving components of the bow accounting for approximately 15% energy loss. As a result, a compound bow will deliver almost as much energy to launch the arrow as was imparted by the archer and stored in the limbs during the drawing of the bow to a single full draw. Using standard compound bows that require one single draw, arrow velocities attained by an archer of greater strength only provide an accuracy in hunting at approximately 20 yards or less. To increase the arrow velocity using a standard compound bow requires that the mean draw force be increased beyond the typical strength capabilities of most archers. Less powerful archers including most women and children are unable to generate the forces necessary during the draw of the bow to result in arrow velocities required for long distance accuracy desired in hunting and in target shooting.
Several approaches have been tried to provide a bow that requires a lower draw force or that store the draw energy with various types of springs. Most of these devices, however store the energy from only a single draw of the draw string, and those devices that provide for a plurality of draws are either cumbersome or require external hydraulic systems to generate the draw energy. One bow has been described which provides a means for improving or modifying energy storage. U.S. Pat. No. 4,471,747 issued to Nishioka stores preloaded energy in a bow which can be recovered at a later time. The mechanism only stores energy of a single pull, however. In Nishioka's first embodiment he describes a device that stores energy of one draw in the limbs by latching an eccentric cam in the let-off position. To launch the arrow, the draw string is pulled back a small amount further to unlatch the latch. In Nishioka's second embodiment he describes a device that again stores energy of one draw in the limbs, although the bow is further equipped with a fluid pump which pushes a piston directly against a pivoting limb as the bowstring is held in its extented position in the let-off region. This bow requires an outside hydraulic source of energy that produces a pumping action on a fluid cylinder to flex a pivoting lower limb. Such hydraulic systems tend to be prone to leakage, are temperature dependent, and are less desirable than standard mechanical systems. The hydraulic pumping action described by Nishioka could not be performed with a let-off of force as found in standard compound bows. Nishioka's bow would thereby require an excessive number of pumping steps or an extremely high draw force for a small number of pumping steps.
Another approach for storing energy in a bow is described by U.S. Pat. No. 4,757,799 issued to Bozek. Bozek requires that two strings be pulled in order to launch an arrow, a bending bowstring and a launching string. Not only is it awkward to pull back two strings at the same time, one of which contains an arrow attached, but the force of each consecutive pull becomes harder than the previous one. Bozek's design further contains problems as the archer releases the two strings related to the return of the two strings to their starting positions resulting in undesireable bow function. The presence of the two strings that are required to be drawn back at the same time by the archer will also negatively affect the accuracy of arrow launch.
Colley describes in U.S. Pat. No. 5,054,463 a bow with a cam located near the riser to provide a let-off or reduced holding force at full draw. Colley further describes a flat wound coil spring that stores the energy of a single draw of the draw string and delivers it back to propell the arrow.
U.S. Pat. No. 3,987,777 issued to Darlington describes a force multiplying type of archery bow with eccentrically mounted pulleys at each end of the handle and provides a reduced holding force at the end of the single draw.
Bixby describes in U.S. Pat. No. 4,989,577 a bow with limbs attached to the handle with a pivot that is spring loaded. Drawing back the bowstring stores energy in the spring but only a single draw of energy is stored.
Several other bows have been described by Colley (U.S. Pat. No. 4,903,677), Jarrett (U.S. Pat. No. 4,512,326), Guzzetta (U.S. Pat. No. 4,756,295), and Boissevian (U.S. Pat. No. 5,150,699) that comprise coil springs, levers, and pivots to provide a significant reduction in draw force or let-off of force in the fully drawn position. All of these devices involve a single draw of the draw string and hence can only store the energy of a single draw.
In U.S. Pat. No. 5,455,139, Bybee describes a cross bow that contains fluid cylinders that push against the middle of bow arms to create increased tension in the bowstring. The system employs an additional hydraulic system to generate the additional energy. Application of Bybee's system to an archery bow or compound bow would create an exceedingly high holding force at full draw since Bybee could not combine any let-off means with the design, and requires a stock assembly to hold the draw string.
Other factors that affect the speed and accuracy of standard compound bows can be related to the cam shaped pulleys located on one or both of the bow limbs of standard compound bows requiring a single draw. These cam-shaped pulleys are usually larger than a small round pulley and hence have a somewhat larger mass. The mass can influence the speed of the bow limb as it returns from its full drawn dynamic state of deformation back to its resting state, making it slower and making the bow limb vibrate more than desired. In a typical compound bow it is also necessary that each of the two bow limbs be constructed similarly such that each bow limb provides similar deformation during the draw of the draw string and return back to their starting position in a similar manner. Attempts have been made to resolve some of these issues however none of the prior art inventions provide a plurality of draws of a single draw string using a mechanical means and launch an arrow using the same draw string.